1. Field of the Invention
The invention relates to storage of information and more particularly relates to use of an electron beam to alter properties of a thin film in order to store information in form of binary code on or in the film.
Specifically, the invention relates to storage and retrieval of information by selectively altering a film's surface potential and electron diffraction characteristics locally through selectively varying positions of associated mobile charged particles. These changes in surface potential and diffraction characteristics are detected by the invention's readout means.
2. Description of the Prior Art
Storage of information by altering some measurable property of a physical surface is old. Virtually all properties of every surface capable of reasonably certain measurement has been utilized for this purpose. A few early examples are marks on stone, physical arrangement of sand in a pattern, and this printing.
The advent of computers made necessary the storage of information in binary, or base two, format. Computers use binary arithmetic for most calculations. As they developed, need emerged to store vast quantities of information in binary format. Many systems were developed to meet this need, and most of them were quite simple. Simple solutions were possible because binary arithmetic uses only two numbers, "0" and "1". A single binary numbr, either "0" or "1", is called a "hit". Any information storage medium capable of assuming two measurably different states can be used to store binary bits of information. One good example of such a medium is the common IBM punch card. Information is stored in the card by punching a hole in the card at a particular address.
Digital computers were denominated "electron brains". Shortly thereafter, binary information storage units became known as computer "memories". As the state of the art in memory construction progressed, greater quantities of binary bits could be stored in smaller memories. Two classes of memories emerged. One of these is "read only" memory (ROM). In this type of memory once information has been stored it can only be used, not erased or altered. Usually ROMs are created by causing some irreversible change in the material comprising a computer's memory. Two examples of ROMs are IBM punch cards and punch paper tape. Here physical holes are punched in paper by mechanical means. Other methods used by ROMs include burning small holes in plastic with light or electron beams and use of such relatively intense energy beams to alter a measurable characteristic of some material, e.g. resistance of selenium. Early computers used this type of memory in the form of physically arranged wires between terminals. ROMs are often used as archival memories for long term storage of data because they cannot easily be erased.
The other class of memory developed is eraseable memory (hereinafter "memory"). Eraseable memories store the same binary formated information as ROMs, but in them this data may be erased or altered by the computer or its operator. Operationally, eraseable memories change the observable state of memory material used to store information from the state representing "0" to the state representing "1" and vice versa. Both types of memory can be random access memories (RAM) and allow any desired bit of information to be located and individually read by means of an "address". The address specifies the desired bit's physical location in the memory. Examples of eraseable memories are manifold. In the beginning, positions and states of electric switches or relays were used to store bits of information. A switch's "on-off" operation is compatible with the "0"-"1" structure of binary numbers. As bit density increased, information was stored in magnetic fields impressed by pulses of electricity onto small ferrite tori or "cores". This memory was used in first and second generation computers and was called "core memory". Although "core" is now a general term for a computer's main memory, ferrite core memory is still used extensively.
Memories are further classified by speed, or, the length of time required to find or "access" any specified bit of information. Core memory is fast. Newer core memories use thousands of microscopic semiconductor switches. Their speed is limited by the speed of electric signals interconnecting their parts. Slightly slower is "drum" memory. Here bits are stored in magnetic fields impressed on a ferrite coated surface of a revolving high speed drum. "Drums" provide fast external memory usually not physically contained within a computer's central processing unit. Drum memory stores, or "writes," and detects, or "reads", magnetically encoded bits with a plurality of tape heads mounted just above the drum's revolving surface.
Slower memories include magnetic discs and tapes operating on principles similar to drum memory. Finally, cathode ray tubes (CRTs) have been used as "storage tube" memory. Storage tube memories operate by bombarding a given location or "address" on the tube's coated glass front surface with a beam of high intensity electrons. These change the electrical charge deposited on the dielectric coating. A small charged spot is thus created. If the electron beam's current is reduced and it is passed over this spot, voltage in an associated circuit changes, allowing the bit to be read. A television camera tube works in a similar manner, except there the tube's "spots" are activated by photons focused by a lens on a photoelectric surface rather than by a high energy electron beam. Unfortunately, all the above discussed memories have many drawbacks. Slow, or "peripheral" memories, such as magnetic disc, tape and drum have moving parts that are subject to wear. Further they are only capable of very low information storage densities. Finally these memories are fragile, physically heavy, bulky, and consume large amounts of power per bit of memory stored.
One parameter for measurement of memory efficiency is the amount of energy required to record and read a single bit of data. Presently, energy intensive methods must be used to alter the state of material used to record information in memory. The most advanced present memories utilize light or electron beams to alter resistivity of material such as selenium.sup.(1) or to physically burn holes or cross link molecules in a thin film deposited on a conducting substrate. .sup.(2) FNT .sup.1 (1) (it requires 1.38.times.10.sup.-11 cal. to change the resistance of a 100.times.100 angstom spot). FNT .sup.2 (2) (6.27.times.10.sup..times.10 cal. and 10.sup.-12 coulombs, respectively, for a 10.sup.4 angstrom square spot.)
The power required to write, store, access and read a bit of information and the physical size of the area required to store a bit of information in memory are two important parameters of any memory system. Magnetic fields, as employed by the memories discussed above, require a relatively large area to store a single bit of information. Bit size in optical memory systems is limited fundamentally by the shortest useable wavelength of light. The smallest obtainable bit size is of the order of 1 micron. Thus, the greatest density obtainable using such a system is on the order of 10.sup.5 to 10.sup.6 bits per square millimeter. Increasing density by several orders of magnitude requires the use of electron beams to write and read the information. Electron beams are not, at least theoretically, limited in spot size. They are, however, limited by the beam current and its associated electronic's signal to noise ratio. Ability to control precise focusing and deflection of an electron beam and the amount of energy contained in such a beam are also considerations limiting the bit size of electron beam addressable memories. A discussion of the state of the art in this field is found in A. T. Miller, Electron Beam Fabrication, Solid State Technology, July, 1973, at pages 25 to 29.
An electron beam used to write data onto a surface must initiate some detectable change in or on the surface so information stored therein may be read and used. One means of writing a bit using an electron beam consists of melting, vaporizing or decomposing a hole through a thin film. Unfortunately, this system results in a non-erasable or archival memory and additionally requires a great deal of energy per bit. A high intensity electron beam can be utilized to crosslink molecules in a thin polymer film. This method uses less energy than required to melt a hole in the film, but still uses a relatively large amount of energy per bit and is non-erasable, resulting in formation of a ROM. Another, state of the art, electron beam addressable memory is now under development. This memory makes use of large difference in electrical conductivity existing between two modifications of some materials, such as selenium. Resistivity of red, crystalline selenium is on the order of 10.sup.15 ohms per centimeter, while resistivity of gray, hexagonal, so-called metallic, selenium is only twelve micro ohms per centimeter. The red form can be converted into the metallic, more stable form by heating with an electron beam. Conversion begins to take place at about 50.degree. centigrade, but can be inhibited by addition of a small percentage of sulphur so it occurs at higher temperatures. The reaction is exothermic, it gives off heat. Selenium can be deposited on a conductive substrate and an electron beam used to heat and convert the red form to the metallic form. The conductive bit, thus created can be read by the same electron beam operating at a reduced beam current.
Again, this process requires that a large amount of thermal energy be obtained from interaction of an electron beam with the storage medium. Although this energy is considerably less than the amount required to melt a film, it still is excessive when compared with the present invention. Additonally, the exothermic nature of the conversion reaction tends to cause bit size to become larger than beam diameter. Also, as the selenium film is reduced in thickness, some electrons will penetrate through it to heat the substrate. This, in turn, will heat the selenium and cause bit diameter to grow even larger. As in the other systems discussed above, it is not possible to erase a written bit because the metallic form of selenium is not readily reconverted to the red form.
The closest approach to the present invention existing in the prior art known to the inventor is an electron beam addressable memory developed by the Stanford Research Institute, which was recently delivered to the Avionics Laboratory of Wright Patterson Air Force Base. This electron beam addressable memory, according to reports (Modern Data, May 1974 at page 14), apparently uses a silicon dioxide layer on silicon as its memory material. The electron beam, reportedly, can write on, read from or erase this silicon target. This memory is built by forming a layer of silicon oxide on a silicon chip, then photolithographically etching a matrix through the oxide layer to form a plurality of silicon oxide insulating islands surrounded by conductive silicon channels. As an electron beam is swept across this matrix its intensity is modulated to place negative charges on these silicon oxide islands in a well known manner. The signal is then read by conventional storage tube methods described by Kazan (see reference Infra).
Unfortunately, data density in this memory is limited to not more than 10.sup.6 bits per square millimeter by the state of the art in photolithography. Additionally, the storage medium is difficult to produce and most, though not at all, of the readout methods used to detect the charge on the silicon oxide islands alter or destroy the charge during the readout operation.